Our objective is to understand the relationship of various cell states and the transitions between them as reflected in the control of histone and chromatin biosynthesis. Currently we are investigating the transition of cells from the cycling state to the quiescent state, using hamster ovary (CHO) cells that can be synchronized. When S phase CHO cells are put into media that leads to the quiescent state, S phase continues at an undiminished rate although protein synthesis begins to significantly diminish. We are using this system to investigate and characterize changes involved at the protein, mRNA, and gene level during this transition. One major question concerns the relationship of the cycling-quiescent transition to the reverse transition, in terms of the expression of oncogene as well as other genes. Present studies are focusing of the mRNA for H2A.Z, a basal histone the synthesis of which is not linked to DNA replication but is 20 fold lower in quiescent relative to cycling cells. One goal is to isolate factors that regulate transcription during this transition. Differential scanning calorimetry is also being used to investigate apparent differences in chromatin topology found between cycling and quiescent cells and the effect of antitumor agents on this parameter.